1. Field of the Invention
This invention relates to the field of miniature multileaf collimators for use in shaping a radiation beam used in stereotactic radiosurgery and radiotherapy. More particularly, the present invention relates to the use of a computer-controlled miniature multileaf collimator capable of shaping a radiation beam for use in optimal small field treatment of nonspherical lesions by radiosurgery and radiotherapy.
2. Description of the Prior Art
In the art of stereotactic radiosurgery, patients are bombarded with radiation beams aimed to deliver a lethal radiation dose to a small tumor or lesion in the brain with minimum dose exposure to normal tissue surrounding the tumor or lesion. Linac-based stereotactic radiosurgery is a technique that focuses a circular radiation beam of x-rays to deliver a single fraction of high dose to a precisely defined spherical target volume through multiple noncoplanar arcs that simulate isotropic radiation. Prior art methods of stereotactic radiosurgery use multiple overlapping spherical treatments to treat nonspherical target volumes. This method has many drawbacks, including significantly increased treatment times, and dose inhomogeneity in the target volume. Such inhomogeneity problems are discussed in Nedzi, L. A.; Kooy, H. M.; Alexander, E.; Gelman, R. S.; Loeffler, J. S. Variables Associated With the Development of Complications From Radiosurgery of Intracranial Tumors, Int. J. Radiat. Oncol. Biol. Phys. 21: 591-99, 1991.
In the past, attempts in the prior art to minimize the dose inhomogeneity problem have led to underdosing near the edge of the target volume just outside the overlapping region of the successive spheres. Such underdosing problems are discussed in Bova, F. University of Florida Stereotactic Radiosurgery Program (Abstr.). SSII 4: AAPM Annual meeting, Med. Phys. 16:511, 1989.
Other prior art devices have attempted to more precisely shape the radiation beam to conform to the geometry of the target volume by using four rectangular blocks in combination with a conventional circular collimator. Such a device is described in Leavitt, D. D.; Gibbs, F. A.; Heibrun, M. P.; Moeller, J. H.; Takach Jr., G. A. Dynamic Field Shaping to Optimize Stereotactic Radiosurgery. Int. J. Radiat. Oncol. Biol. Phys. 21:1247-55; 1991. The variety of nonspherical beam shapes achievable with such a device is extremely limited due to the fact that only four straight edges can be used in conjunction with the spherical configuration of the circular collimator to form a dynamically shaped beam.
Another drawback of the prior art multileaf collimators is that they are designed for large field treatments in the range of 40 centimeters by 40 centimeters. Such collimators have leaf widths of 1.0 to 1.25 centimeters at isocenter, the beam-shaping capabilities of such collimators are too coarse for radiosurgical use. An example of a prior art multileaf collimator intended for delivering radiation to a large treatment area is disclosed in. U.S. Pat. No. 5,160,847 to Leavitt, et al. ("the Leavitt patent").
Prior art multileaf collimators lack leaf position-indicating devices that are sufficiently precise and accurate for use in small field treatment applications. Prior art multileaf collimator position-measuring devices have included potentiometers, as disclosed in the Leavitt patent. Potentiometer based position indicating devices are unreliable for measuring leaf movement of less than 0.2 millimeters.
During stereotactic radiosurgery, the radiation source is moved along an arc defined by a gantry. The point at which all radial lines defining this arc intersect is known as the isocenter of the radiation beam. Such movement of the radiation source is customarily employed to totally irradiate a tumor or lesion. Due to the nonuniform shape of tumors and lesions, it is desirable to change the beam geometry as the radiation source moves along the arc. This is known as dynamic shaping. Many prior art multileaf collimators have limited dynamic shaping capability because all of the leaves cannot be moved at one time. In order for dynamic shaping to be effective, the multileaf collimator must have the capability to reposition the leaves quickly.
The present invention overcomes the drawbacks of the prior art by providing a computer-controlled miniature multileaf collimator capable of dynamically shaping beam geometry to closely conform to nonspherical targets. Additionally, the present invention is designed for small field treatments, in the range of 6 centimeters by 6 centimeters at isocenter. The present invention further comprises position-indicating means sufficiently accurate and precise for use in small field treatments.